With automobile telephones or portable telephones becoming more and more widely used, a problem has been revealed in that the conventional analog mobile telephone system tends to be short on radio channels. While it is true that the conventional analog mobile telephone system can be easily configured, it has a disadvantage in that its allocation of one frequency to one channel entails inefficiency in the use of frequencies. On the other hand, with time division multiplexing effected in the digital mobile telephone system, a single frequency can be shared by a plurality of channels, thereby improving efficiency in usage of frequencies. It is also to be noted that the digital mobile telephone system can provide advanced services other than conventional voice call service.
FIG. 1(A) illustrates a service area of the conventional digital mobile telephone system of a cellular type.
Referring to FIG. 1(A), a service area of the mobile telephone system is divided into cells or sectors P having, for example, a hexagonal shape, each of a plurality of base stations D, E, F being provided to correspond to a plurality of sectors. In the illustrated example, a base station D is provided in the center of the three hatched sectors P, the base station D covering the three sectors in different frequencies. Likewise, each of the base stations E, F cover three sectors. In such a digital mobile telephone system of a cellular type, it is possible to avoid interference among the sectors by using different frequencies in different sectors. Typically, a sector has a scale of about 500 m-5 km.
FIG. 1(B) schematically illustrates a signal format used in the system of FIG. 1(A).
Referring to FIG. 1(B), a signal having a frequency f.sub.A transmitted from a base station--for example, the base station D--contains three channels TCH.sub.1, TCH.sub.2, CCH obtained as a result of time division multiplexing, of which three channels the channels TCH.sub.1, TCH.sub.2 are used as communication channels, and the channel CCH is used as a control channel for line connection control, etc. Likewise, a signal having a frequency f.sub.a received by the base station D contains channels obtained as a result of time division multiplexing: namely, communication channels TCH.sub.1, TCH.sub.2 and a control channel CCH. While the communication channels TCH.sub.1, TCH.sub.2 carry independent speech signals, etc., the control channel CCH is provided to be shared by the channels TCH.sub.1, TCH.sub.2. With such time division multiplexing, the digital mobile telephone system shown in FIG. 1(A) accomplishes more efficient use of frequencies than the analog system.
FIG. 2 illustrates the configuration of the digital mobile telephone system covering the service area of FIG. 1(A).
Referring to FIG. 2, the system comprises: a mobile terminal 1 moving about in the service area of FIG. 1(A); radio base stations 2 corresponding to the base stations D, E, F of FIG. 1 and communicating with the mobile terminal 1 using a radio channel; and a switching station 3 connected to the radio base stations 2 via wire or radio lines and connected further to the public telephone line, the mobile terminal being connected to the public telephone line via the switching station and the radio base station. In such a digital mobile telephone system, there is a need to change the frequency used in communication with the base station, when the mobile terminal 1 moves from one sector to another. For this purpose, the switching station 3 is equipped with a control apparatus 3b besides an ordinary switching apparatus 3a. On the basis of the field strength of the radio wave signal transmitted from the base station and detected by the mobile terminal 1, the control apparatus 3b changes the setting of the frequency used by the mobile terminal so that the frequency transmitted from the base station and producing the greatest field strength may be set to be used.
More specifically, in the case that the switching station 3 is connected to a radio station BS-D via a line CH, and the mobile terminal 1 receives messages on the public telephone line via a transmission TX1 from the base station BS-D, the mobile terminal 1 measures the field strength of the transmission TX1 and determines whether or not the measurement result exceeds a predetermined threshold level. If it is found that the field strength exceeds the predetermined level, the mobile terminal 1 maintains its line connection with the base station BS-D; if it is found that the field strength is below the threshold level, the terminal 1 measures the field strength of transmissions from other radio base stations BS-E-BS-H searching for the base station producing the greatest field strength. The identity of the base station determined to be used in this search is reported to the base station BS-D via a transmission line TX2; and the information thus reported 35 is forwarded from the base station BS-D to the control apparatus 3b of the switching station 3 via the line CH. Thereupon, the control 3b switches the line for use in connecting with the mobile terminal 1 from the line CH which runs by way of the base station BS-D to, for example, a line CH' which runs by way of the base station H, i.e., the base station producing the greatest field strength. By thus selecting the base station used in communication with the mobile terminal 1 on the basis of the field strength such that the greatest field strength may be obtained, it is possible to maintain a stable line connection whichever sector in the service area the mobile terminal is in.
There is proposed, for such a conventional digital mobile telephone system as shown in FIGS. 1(A), 1(B) and in FIG. 2, a so-called overlay configuration shown in FIG. 3 designed to further improve efficiency in the use of frequencies. In the overlay configuration, overlay regions Q, covering a range substantially smaller than the size of the sector, are formed to correspond to each of the base stations BS-D, BS-E, BS-F, etc. For each base station, the overlay region Q is covered by a small-power transmission at a frequency different from the frequency characterizing the sector, with the result that the frequency band of the system increases substantially. By forming the overlay regions Q using the frequency of the same frequency range for every station, the system's efficiency in the use of frequencies is greatly improved. Since the overlay regions Q are independent of each other, no interference arises even if the same frequency is used from one overlay region to another.
When operating the mobile telephone system having such overlay regions Q, various problems needing a solution exist that are related, for example, to frequency switching control of the mobile terminal. For example, while the conventional cellular system enables selection of the base station producing the greatest field strength around the mobile terminal, as described earlier, the mobile terminal frequency switching control employed in the conventional cellular system is not effective in case the mobile terminal moves in and out of the overlay region, since the field strength in the overlay region is smaller than that in the sector covered by the same base station. There is a need, when operating the digital mobile telephone system having an overlay configuration, to establish a switching control method for selecting the frequency used in communication between the mobile terminal and the base station, which method is effected when the mobile terminal moves in and out of the overlay region.